indeed."Is there a measurable limit to how fast a sequence of notes can be executed on a diatonic"
define "play" - Remember the guy who claimed the 'worlds fastest harmonica player' title for himself on guiness or wherever by simply running the harp back and forth nonsensically?
I think to know the current upper limitations on speed playing, check the music by John Popper, Jason Ricci, Pat Ramsey, Charlie McCoy and some bluegrass players like Cara Cooke, etc.
perhaps a more useful question is what kind of notes can someone play. 16th notes? i expect yes. 32th? dunno. but then, i don't know if other instruments can do that either.
-- We make a living from what we get, but we make a life from what we give. - Kathy Moser